1. Field of the Invention
The present invention relates to integrated circuit devices and more specifically to field effect devices such as field effect transistors (FET) for use in integrated circuits.
2. Description of the Prior Art
In manufacturing transistors, reoxidation has been used in 5 μm to 1.2 μm technologies to improve transistor lifetimes and gate oxide reliability due to higher fields occurring at the etched polysilicon transistor edges. For example, U.S. Pat. No. 4,553,314 teaches using reoxidation to manufacture semiconductor devices. Typically, 3 μm and 5 μm technologies use reoxidation thicknesses from about 1200 Å to about 2500 Å depending on the particular device. In 1.5 μm and 2 μm technologies, reoxidation thicknesses from about 500 Å to about 1,000 Å are used.
In 0.8 μm technology, however, the reoxidation process has been discontinued because the lifetimes of transistors currently manufactured without the reoxidation process is better than with the reoxidation process. Such a situation is caused by the formation of asperities on the underside of the polysilicon layer of the transistor during the reoxidation process. These asperities are of little importance until the gate oxide thicknesses are reduced to below 200 Å as used in submicron technology. At this point, the asperities become a contributor to the increased field at the transistor edge and of hot carrier injection (HCI). These asperities are caused by (1) oxidant diffusion along polysilicon grain boundaries creating single crystal silicon protrusions and (2) oxide thicknesses under the polysilicon edge increasing during reoxidation, causing polysilicon grain boundary slip to occur and creating multiple edges, which results in an overall increase in angle geometries.
In addition, moving to device geometries below 0.8 μm technology has resulted in marginal lifetimes of the transistors. Thus, it is desirable to have a gate structure that has an increased lifetime during reoxidation under the gate edge but without the asperities caused by presently used reoxidation processes.